It's not easy to get clear information about Lexus tpms systems, but after a few hours with google I think I have figured out the following.

1) The 2012 RX350 system is a direct (pressure) system not an indirect (wheel sensor revolutions) system.

2) The system triggers "low" when the pressure senses a 25% drop from the "set" pressure. Example you set the system at 32 psi, it triggers low at 24 psi (8 psi is a 25% drop).

3) My RX does not indicate the position of the low tire (for which I am grateful) therefore I can rotate wheels front to back (or anywhere else) and not have to reset/reinitialize/reprogram or re anything when I rotate every 5k miles.

If you have a good level of understanding of the specific sytem or tpms in general, please post if I have this right or not.

Please don't GUESS, I can guess as well as you, I am looking for facts, not my neighbor has one & his barber's mechanic said....

Thanks!

 

Hi Froggy,
As to life in general, sometimes a guess can lead to an "ah-ha moment" which can lead to the facts. Engineers even have a time-tested, tried and true system called WAG, which stands for "Wild A** Guess, this system can even pass very close scrutiny, sometimes.
This has been a joke, of sorts.
As to TPMS systems, I wouldn't venture to guess, but everything you've said makes perfect sense to me. I think you have it right.
Ray

 

Fair enough Ray, a well articulated post.

What I am aiming at is to "hack" or "game" the tpms a little so it works to a higher more useful level than designed, I just need to know that it works they way I think it does, otherwise, my hack is for naught.

If it works as above laid out I would suggest this.

My target WARNING pressure is 30 psi. I want it to tell me if a tire drops to 30 psi. I want to set the actual pressures at 32 psi. So I want a warning when I lose 2 psi.

I think if I put the pressures at 40 psi and then do the reset procedure, the government mandated (always knows best for us or at least how to control us) warning will go off at 30 psi - a 25% drop. 40 - 30 = 10. 10 is 25% of 40.

Everyone follow so far?

Then I go around with my gauge and set them all at 32 psi.

Now when the monitor sees 30 psi anywhere it goes OMG! this guy has lost 25% of his pressure must warn him!!!

So I don't have to ride around using more gas, poor handling, and wearing out the tires WAITING for the pressure to drop to 75% of 32 which is what, 24 psi, which IMO is dangerously low at that. And has been for months probably.

The manufacturers (Lexus) have no choice but to build stuff like airbags, tpms, pollution controls to meet the letter of the law that idiots in congress dream up (to look like they are doing something and get reelected).

The unintended consequence of this tpms law is that the "average" driver thinks his tire pressures are all hunky dory unless the warning light goes on. So all the people that used to check tire pressure "once in a while" now wait for an idiot light to tell them to do it. And the ones that did not know what a tire pressure gauge is, are still as clueless as ever. So net, across the country, there are very possibly MORE folks driving around on under inflated tires than before the law.

Don't get me started on the other law we have in CA (maybe other states) that REQURES anyone in automotive services to set the tire pressures to "official" pressures even if you show up for new wiper blades.

Rant off.

In any event If a tech can confirm that the tpms runs as I think I will go ahead & try it.

 

Just thinking out loud...

A tires pressure fluctuates 1 PSI for every 10 degrees of temperature change. This is true for any gas, including nitrogen. If the TPMS trigger was able to detect a narrow 2 PSI range, the alert would be going off all the time. i.e., If you set your tire pressure at 32 PSI during the day at 80 degrees, the alarm would likely go off in the evening when it was below 60 degrees. Same holds true in the other direction of temperature shift.

There is likely well thought out logic for the current range in the TPMS. Tires on the road heat up due to both the pavement heat and the friction caused by flexing of the sidewall. The spare doesn't see this. So frequently you are going to have a greater than 2 PSI differential from tire to tire. I suspect most consumers would object to seeing the TPMS light all the time and finding nothing wrong would just begin to ignore it.

I believe our TPMS is designed to alert drivers to a significant pressure loss needing prompt attention, not to monitor small pressure fluctuations.

Ideally, I'd like my TPMS system to display each tires PSI on the NAV screen upon starting the vehicle.

 

Thanks for adding Jim,

For my idea to work, the following is necessary. Initial setup is on a cool day before the car is driven. So the pressure really only has one way to go up, never goes below the cool setting unless I park on a iceberg. Reset required betwwen summer vs winter of course. THIS IS KEY.

The system never reports OVER pressure so we can ignore pressure gain.

I could expand the range, maybe 2 psi is a little tight for the built in accuracy of the pressure modules.

My 2012 space saver spare is already 12 - 20 psi over the 4 full size tires with no alarm going off, so I have to assume some how or other the module is not looking at differentials between tires, maybe just looking at raw pressure per each & waiting for the 25% drop, in which case my hacked system is ok. Or the system ignores the space saver spare, who knows? Lexus is certainly not giving us that info easily.

It may be that using google & forums I will never get a fact based explanation on how 2012 system works, just little bits and pieces of anecdotal info. In which case I will just play around with the pressures. I guess there are about a dozen solutions to the federal mandate among the auto mfgs, and then there are year to year changes by maker (such as Lexus) so it's a bit daunting to get clear info on all that.

I do think you are right, 2 psi is cutting it too close.

 

Froggy, here's something to ponder on, the 450h reports actual tire pressure. It doesn't indicate which tire is which, but does have 4 values in PSIs. It is possible that the system is designed to trigger on specific pressure threshold, rather than a percentage drop. I have not tried to set it off, but I'm looking forward to your findings. Please proceed with the experiment.

Also, the spare does not get reported... unless one is driving on a snow set w/o TPMS, then the system picks up the phantom 60PSI tire.